Abstract

Transcranial direct-current stimulation (tDCS) is a noninvasive brain stimulation technique that has been successfully applied for modulation of cortical excitability. tDCS is capable of inducing changes in neuronal membrane potentials in a polarity-dependent manner. When tDCS is of sufficient length, synaptically driven after-effects are induced. The mechanisms underlying these after-effects are largely unknown, and there is a compelling need for animal models to test the immediate effects and after-effects induced by tDCS in different cortical areas and evaluate the implications in complex cerebral processes. Here we show in behaving rabbits that tDCS applied over the somatosensory cortex modulates cortical processes consequent to localized stimulation of the whisker pad or of the corresponding area of the ventroposterior medial (VPM) thalamic nucleus. With longer stimulation periods, poststimulation effects were observed in the somatosensory cortex only after cathodal tDCS. Consistent with the polarity-specific effects, the acquisition of classical eyeblink conditioning was potentiated or depressed by the simultaneous application of anodal or cathodal tDCS, respectively, when stimulation of the whisker pad was used as conditioned stimulus, suggesting that tDCS modulates the sensory perception process necessary for associative learning. We also studied the putative mechanisms underlying immediate effects and after-effects of tDCS observed in the somatosensory cortex. Results when pairs of pulses applied to the thalamic VPM nucleus (mediating sensory input) during anodal and cathodal tDCS suggest that tDCS modifies thalamocortical synapses at presynaptic sites. Finally, we show that blocking the activation of adenosine A1 receptors prevents the long-term depression (LTD) evoked in the somatosensory cortex after cathodal tDCS.

Effects of tDCS on LFP evoked in the vibrissa S1 area of the somatosensory cortex. (A) Experimental design illustrating the electrode locations for tDCS and the recording micropipette. Both anodal and cathodal tDCS currents were applied between the active (red circles) and the reference (Ref.) electrodes. Air puff stimulation of the contralateral whisker pad is shown as well. (Inset) Schematic sagittal view of the recording site at which LFP corresponding to vibrissa was mapped, the tungsten electrode (Rec.) was attached to the skull, and the window was cut in the parietal bone and subsequently covered with dental cement. (B) Representative examples of LFP evoked in the vibrissa S1 cortex by air puff stimulation of the contralateral whisker pad in control situation (black recording) and during the application of anodal tDCS (red recording) or cathodal tDCS (blue recording). (C) Changes in amplitude (Left) and area (Right) of the N1 component (indicated in B) of air puff-evoked LFP in the presence of anodal currents (red histograms) or cathodal currents (blue histograms) at increasing intensities. n = 5. *P < 0.005; **P < 0.01, one-way ANOVA. (D) After-effects of tDCS. LFP evolution after anodal tDCS (red) and cathodal tDCS (blue). ***P < 0.001, repeated-measures ANOVA. Error bars represent SEM.

Effects of tDCS on classical eyeblink conditioning using a trace paradigm. (A) Experimental design. Two habituation sessions (H1 and H2) and 10 conditioning sessions (C1–C10) were carried out. Conditioning sessions consisted of 66 trials (6 series of 11 trials each) separated at random by intervals of 50–70 s. Of the 66 trials, 6 were test trials in which the CS was presented alone. Anodal tDCS was presented during series 2, 4, and 6 of C2 (in red), and cathodal tDCS was presented in these same series during C8 (in blue). (B) The conditioning paradigm (CS and US presentations) and representative orbicularis oculi electromyographic (O.O. EMG) recordings collected from the same animal during the C2 session from a control series (conditioning series 1, traces in black) and during anodal stimulation (trace in red) and during the C8 session from a control series and during cathodal stimulation (trace in blue). The presence of conditioned responses (CR) is indicated for the anodal-stimulated animal during the C2 session and for the control animal during the C8 session. (C) Evolution of learning curves for a control group of animals (gray squares and line) and the experimental group. The experimental group received anodal tDCS during C2 and cathodal tDCS during C8. n = 6. P < 0.05, one-way ANOVA. (D) Evolution of the percentage of conditioned responses during sessions with anodal tDCS (C2, in red) and cathodal tDCS (C8, in blue). Series during which tDCS was applied (S2, S4, and S6) are indicated by a gray bar. P < 0.05, Mann-Whitney test. Error bars represent SEM.